This invention relates generally to offset printing and more particularly to a method and system for registering in an offset printing press and to a plate bending machine for efficiently practicing the registering method.
Heretofore, there have been various methods for registering in web offset printing presses or sheet-fed offset printing presses. In general, however, registering being carried out consistently from the reproduction process step, through the step of making the printing plate and the plate bending step, to the plate mounting step by the so-called pin hole datum or reference method wherein pin holes formed in the printing plate are used as datum references.
More specifically, the steps of forming pin holes, in a printing plate, to be datum references for mounting the plate on a plate cylinder, of bending opposite sides of the printing plate, and of forming register marks for registering on the plate have been carried out with pin holes formed in corner parts of the printing plate as datum references.
By a method wherein all process steps are carried out in this manner with pin holes as datum reference, registering is not necessarily correct when the printing plate is mounted on a plate cylinder of the printing press because of "play" and/or "wear of the pin holes". This deviation is of a magnitude exceeding the limits within which it can be rectified in the printing press in many cases, whereby automatic correction has been impossible in actual practice. The reason for this is that, in addition to the positioning in the rotational direction and the lateral direction of a printing plate, registering involves the factor of twist which means the plate is mounted obliquely on the plate cylinder.
More specifically, deviations in the rotational direction and the lateral direction of the plate can be automatically corrected within a relatively wide range of the order of .+-.2 mm, but the twist can be corrected only within .+-.0.3 mm because of mechanical restrictions. When there is an error in the mounting of the plate exceeding these limits, it becomes necessary to remount the plate on the plate cylinder after rebending end portions of the printing plate.